I don't think they expect everyone to report bugs, because not everyone will run into the same issues or cares about reporting them. I'm sure they've considered the number of keys they need to hand out in order to get a good amount of feedback.
Patience guys.
I have my doubts that it'll be the entire DLC.
They've already said that it will be the complete DLC, including Xbox LIVE achievements. And they've said that those selected for testing will not have to pay for the DLC. And I think you're right that they have probably done a sufficient amount of internal testing, but I still think anyone lucky enough to be selected for testing had better give them as much feedback as is possible/reasonable.